Recently, card-type memory devices such as IC cards have been brought into practical use. In a card-type memory device, a semiconductor chip is mounted in a semiconductor package. Typically, the semiconductor package has the chip resin-molded on one side and a planar-type external connection terminal on the other side. The chip may be a nonvolatile semiconductor memory device or any other type of semiconductor chip.
FIGS. 9, 10(a), and 10(b) show a conventional semiconductor package for a card-type memory device. FIG. 9 is a cross-sectional view of the semiconductor package, FIG. 10(a) is a perspective view of the chip side of the semiconductor package, and FIG. 10(b) is a perspective view of the external terminal side of the semiconductor package. As shown in FIG. 9, a substrate 1 is formed of resin, which has a thickness of approximately 0.1-0.4 mm, and a semiconductor chip 6 is bonded to the substrate 1 by an adhesive 9. A gold wire 7 provides an electrical connection between a chip connection terminal 2 of the substrate 1 and a bonding pad of the semiconductor chip 6. The chip side of the substrate 1 is molded by a resin 8 that covers the semiconductor chip 6, and an external connection terminal 3 is provided on the other side of the substrate 1. The external connection terminal 3 is electrically connected to the chip connection terminal 2 via a through-hole 4 that penetrates the substrate 1.
FIG. 8 shows a cross-sectional view of the substrate used in the conventional semiconductor package of FIG. 9. (In the different figures, elements that are the same are represented by the same reference numerals and duplicate descriptions thereof are omitted.) The chip connection terminal 2, which is located on the chip side of the substrate 1, is typically plated with soft gold with a purity of 99.9% or higher. The soft gold on the chip connection terminal 2 provides a good connection between the bonding wire 7 and the chip connection terminal 2 because soft gold or aluminum is typically used for the bonding wire that is connected between the chip's bonding pad and the chip connection terminal of the substrate 1. On the other hand, the external connection terminal 3, which is located on the side of the substrate opposite the semiconductor chip 6, is typically plated with hard gold with a purity of approximately 99%. Hard gold is used for the external connection terminal because it offers a greater resistance to damage. Thus, a boundary 5 between the soft gold plating and the hard gold plating is located at a central portion of the through-hole 4. For simplification, FIG. 8 does not individually show the gold plating, nickel plating, and copper plating and foil that are described below. These layers are collectively shown as the chip connection terminal and the external connection terminal in FIG. 8.
FIGS. 11(a) through 11(f) show a typical manufacturing, process for the conventional semiconductor device substrate described above. First, copper foils 24 with an illustrative thickness of about 18 .mu.m are attached using an adhesive to both sides of a resin substrate 1, as shown in FIG. 11(a). The substrate 1 is then drilled to open a through-hole 4, as shown in FIG. 11(b). Next, as shown in FIG. 11(c), the entire substrate is plated with copper so that copper plating 25 is provided on the inner side of the through-hole and on both sides of the substrate. Thus, the copper plating provides an electrical connection between the two sides of the substrate. A photoresist-type dry film is then pasted onto the copper of the substrate and a copper interconnection is formed through exposure of light, patterning, and etching of the copper, as shown in FIG. 11(d).
The copper interconnections are typically formed of both copper foils and copper plating because the copper foils can be used to easily and quickly increase the thickness of the connection by merely pasting them onto the substrate. However, the copper foil cannot be attached to the inner side of the through-hole to complete the interconnection. On the other hand, it is rather difficult to increase the thickness of the connection terminal through copper plating because of the slow progress of the plating process. In cases where a low relative thickness or slow progress are not drawbacks, it is possible to omit the copper foils and use only the copper plating to form the copper interconnections.
In the next step of the manufacturing process, the chip-mounting side of the substrate is entirely masked using a tape or photoresist-type dry film. Bright nickel (not shown) and hard gold are then consecutively plated on the substrate to provide a hard gold plating 3 over the copper interconnection on the external terminal side of the substrate and on the inside of the through-hole 4, as shown in FIG. 11(e). Then, the hard gold-plated external terminal side of the substrate is entirely masked using a tape or dry film. Non-bright or semi-bright nickel (not shown) and soft gold are then consecutively plated on the substrate to provide soft gold plating 2 over the copper interconnection on the chip-mounting side of the substrate and on the inside of the through-hole 4, as shown in FIG. 11(f). The nickel plating is interposed between the copper plating and the gold plating because the intervening nickel layer prevents a slow diffusion of the gold into the copper. In another typical manufacturing process, the order of the gold plating is reversed so that soft gold is first plated on the chip-mounting side and then hard gold is plated on the external terminal side of the substrate.
In such conventional manufacturing processes, one side of the through-hole 4 is closed by the masking during both soft gold and hard gold plating. As a result, air builds up in the through-hole 4 and the plating solution is prevented from flowing through the through-hole 4. Therefore, in some cases, no plating is attained at the central portion of the through-hole 4. Whenever the through-hole 4 has portions that are not plated by either the soft gold or hard gold, the underlying metal layer of copper or nickel is exposed to oxygen and the like. This allows corrosive action to occur in the exposed portion of the through-hole so that a breakage of the interconnection between the two sides of the substrate may result. On the other hand, there is a possibility that the underlying metal layer of copper or nickel is exposed to hydrogen or the like. For example, when the nickel is soluble in water, the nickel is subject to ionization. This allows the solution of nickel to occur in the exposed portion of the through-hole.